If all we had to go on was "I must get on her nerves", then that would be ambiguous. Given the extra information in the first part of yoursentence, no native speaker would be confused about which "must" was meant.

Now there's something that I've never heard, over here. Is it common practice in British English to use the "a" in that sentence? In North America we would say "It is nonsense". Does that shift the meaning, slightly?

Now there's something that I've never heard, over here. Is it common practice in British English to use the "a" in that sentence? In North America we would say "It is nonsense". Does that shift the meaning, slightly?

Now there's something that I've never heard, over here. Is it common practice in British English to use the "a" in that sentence? In North America we would say "It is nonsense". Does that shift the meaning, slightly?

I don't think it shifts meaning John. I would use either happily.

I am mostly with Bob on this. But, I think, using the "a" makes it more directed.
eg
1. Much of Lewis Caroll's work is nonsense, which is what endears it to the imagination.
2. The 10 commandments are quite sensible but to say 'I must not covet my neighbour's wife' is a nonsense.